Waze is a free turn by turn navigation app for the iPhone that uses Open Street maps for its maps and encourages it's community to maintain the maps. The following bookmark will (usually) open Waze and a moment or two later shift the map to the waypoint and drop a pin. From there just tap the pin and "go" to navigate to the general area of the cache. If that not where you want to go but just near by, tap and hold a spot on the map (like a parking lot for instance) and then navigate to there.